The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another so as to house and protect critical elements of the nervous system. In addition, the spine is a highly flexible structure, capable of a high degree of curvature and twist in multiple directions. The most flexible of all the regions of the spinal column is the cervical spine.
The bones and connective tissue of an adult human spinal column are coupled sequentially to one another by a tri joint complex which consists of an anterior disc and the two posterior facet joints. The anterior discs of adjacent bones are separated and cushioned by cartilage spacers referred to as intervertebral discs. The vertebral bones of the spine are classified as cervical, thoracic, lumbar and sacral. The cervical portion of the spine, which comprises the upper portion of the spine up to the base of the skull, includes the first seven vertebrae. The twelve intermediate bones comprise the thoracic vertebrae, and connect to the lower spine which comprises the five lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The vertebrae which make up the cervical portion of the spine are generally smaller than those of the thoracic and lumbar spine.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease are a few of the causes which can result in spinal pathologies for which permanent immobilization of multiple vertebrae may be necessary. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies.
The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determines the appropriate surgical protocol and implantation assembly. The use of posterior plates for stabilization and immobilization of the cervical spine is known. A posterior plate is a narrow elongated plate having a series of spaced holes through which screws may be inserted to fix the plate to the vertebrae. A pair of posterior plates is placed across the lateral posterior surfaces of a set of sequential cervical bones and is secured to the bone by using one screw per vertebra, thereby preventing the bones from moving relative to one another in either the vertical or horizontal planes.
Because the spine is routinely subject to high compression and torsional loads which cycle during movement, one of the primary concerns of physicians performing spinal implantation surgeries, as well as of the patients in whom the implants are placed, is the risk of screw pull-out. Screw pull-out occurs when the cylindrical portion of the bone which surrounds the inserted screw fails. A bone screw which is implanted perpendicular to the plate is particularly weak because the region of the bone which must fail for pull-out to occur is only as large as the outer diameter of the screw threads. It has been found that for pull-out to occur for screws which are inserted into the bone at an angle with respect to the plate, the amount of bone which must fail increases substantially as compared with screws which are implanted perpendicularly with respect to the plate.
An additional concern with screws being implanted in the posterior side of the cervical spine is that there are sensitive and important structures adjacent to the boney structures, such as the lateral masses and the laminae, which, because of their proximity to the implant, may be damaged by insertion or dislocation of screws. In the cervical spine, the vertebral arteries are disposed medially beneath the lateral masses or lamina and comprise critical structures which cannot be compromised. In addition, the facet joints which provide natural coupling of sequential bones together must also be avoided it possible. Avoidance of these bodies has been a critical and ongoing concern with respect to posterior screw insertion. Posterior plates of the prior art have provided little in the way of reasonable or practical solutions for ensuring proper screw insertion.
Posterior screw plate assemblies necessarily include a plurality of screws which are inserted through a single plate. However, if a single screw loosens with respect to the surrounding bone into which it has been inserted, loss of fixation occurs and possible neurological repercussions may result.
One way to avoid the drawbacks of current plate systems has been to use fixation systems that employ polyaxial screws, rods, and hooks. However, while polyaxial screws provide a surgeon with the ability to locate the screws in optimum locations, the ability to do so requires a high degree of skill and experience. Further, to ensure proper placement of polyaxial screws, surgeons typically utilize fluoroscopy for an extended period of time which can expose patients to unwanted radiation.